D&D 5E Last night I dreamt up a campaign setting... help me flesh it out!

[squibbles]

I'm afraid I skimmed a lot of the other long posts, but responding to the OP:

That setup screams post-post-apocalypse to me. There was a horrible disaster, but people are past it now and the world has been rebuilt in the shadow of said apocalypse's consequences. Maybe the undead are only ONE of those consequences. This setup is also perfect grist for adventuring since apocalypses tend to leave behind a lot of valuable merch in miscellaneous underground facilities.

The idea of having one giant walled centerpiece city as @jmartkdr2 suggested makes sense and has lots of antecedents. One that comes to mind, which you might steal from/mine for inspiration, is Green Ronin's The Lost Citadel setting, given its similar walled city surrounded by undead setup. The other thought is, oddly, the barrier cities from Final Fantasy: The Spirits Within, with their technological defenses against incorporeal undead.

Speaking of, it seems like undead apocalypses always go straight for zombies and such. It might be worthwhile to lean more on how incorporeal undead fit into all of this. If a swarm of zombies is attracted to cleric magic, but the city has high walls and mechs to defend it... no big deal. But if cleric magic attracts wraiths and ghosts that can fly over/through the walls and past the defenders, then that ban on clerics is more important. Maybe as a balancing element mechs do better against zombies, while wizards, clerics, and fighters with magic swords do better against ghosts

Finally, the Warlock patron makes me think of HR Giger; a shady pact signing warlock patron probably isn't all sunshine and rainbows with its cybernetic grafting.

 

[Stormonu]

Battlewalker = Warstrider?

Also, I think a Guyver suit for battle-ready Warlocks would be an interesting addition to the campaign.

 

[aco175]

I can see a technology vs nature slant to the campaign world. There can be a few big cities and several smaller towns and farming areas that are protected and large areas of wilderness or wastelands that the undead roam. There can be druids that work for both sides with one being a nature-first slant that helps the undead mostly be opening paths and making it easy to get to the cities.

The no divine magic in cities seems to be opposite of what I expected giving the undead threat. Maybe the kingdom leader is a powerful undead hiding like the emperor in Star Wars not wanting jedi around anymore. Pit the divine against the mechs and somewhere there is friction. There should be a place where divine is welcomed even if it is in the neighboring kingdom.

There should be some sort of underground resistance to the big forces. Maybe this is where adventurers come around in the smaller towns and frontier areas. There could be some less resistance to divine with overlap with druids helping grow crops and maybe a cleric healing the mayor's kid.

Somehow I was getting this idea reading the other's responses.

 

[jmartkdr2]

Battlewalker = Warstrider?

Also, I think a Guyver suit for battle-ready Warlocks would be an interesting addition to the campaign.

Warstrider is a better name, but it's already used in Exalted.